Shock and vibration resistant heater for indirectly heated cathodes of radio receiving tubes



July 26, 1966 P. TURY 3,263,114

SHOCK AND VIBRATION RESISTANT HEATER FOR INDIREGTLY HEATED CATHODES 0F RADIO RECEIVING TUBES Filed Oct. 26, 1960 FL g. 3

INVENTOR. P 'L Tu RY BY wan M CW6 United States Patent 3,263,114 SHOCK AND VIBRATIQN RESISTANT HEATER FOR INDIRECTLY HEATED CATHODES OF RADIO RECEIVING TUBES rs: Tury, Budapest, Hungary, assignor to the firm Egyesult Izzolampa es Villamossagi Reszvenytarsasag, Budapest, Hungary, :1 Hungarian enterprise Filed Oct. 26, 1960, er. No. 65,197 1 Claim. (Cl. 313-346) This invention relates to indirectly heated cathodes for electric discharge tubes, such as radio receiving tubes, and to electric discharge tubes, especially high-vacuum electron tubes provided with said cathodes.

It is well known that the indirectly heated cathodes of conventional electron tubes consist of a cathode body provided with an electron-emitting active material, i.e. carrying a coating of the said active material, or impregnated with the said active material or containing this material in a storage chamber, and a heater element consisting of a metal of high melting point, shortly termed hereinafter as refractory metal and arranged in an internal cavity of the cathode body. The invention relates to cathodes of the aforementioned type and to electric discharge tubes, more especially high-vacuum electron tubes, provided with these cathodes.

It is known that the aforesaid heater elements are usually manufactured from tungsten or another metal of high melting point and consequently the expression refractory metal in the present specification and claims indicates one of the following metals: tungsten, tantalum or molybdenum. These metals are used as heater elements of the cathodes, either in the shape of simple wires, or of singlecoil or double-coil wires. In the majority of cases these wires serving as heater elements are provided with an insulating coating, for example with an aluminum oxide coating which is sintered thereon, but in some cases they are provided inside the cathode body without having an insulating coating on their surfaces.

In connection with my investigations of the existing conventional electron tubes provided with cathodes of the aforesaid type, more especially those electron tubes which are used with their heater elements in series connection, I have ascertained that the heater elements of the cathodes of such tubes do not have a mechanical resistivity such as may be necessary with the modern industrial and other uses of the tubes. The standards required in this connection are becoming increasingly higher and the heater element frequently breaks prematurely because of its insufficient mechanical resistivity, whereby the tubes become useless. Electron tubes and other discharge tubes which are subjected to high mechanical stressing can consequently only be made in certain cases by quite special constructions greatly increasing the cost of the tubes, in order to enable them to be more resistant also in a mechanical respect.

The object of my present invention is to provide such a cathode or tube in which the heater element respectively elements of the cathode have high mechanical resistivity without it being necessary for this purpose to use particularly costly constructions and/or materials. By the use of the cathodes according to my invention, therefore, the tubes provided with these cathodes have a considerably greater mechanical resistivity and consequently can also be used in those spheres where this formerly was impossible.

Th theoretical considerations forming the basis of my present invention are briefly summarized hereinafter as follows:

It is known that in the industry concerned with the vacuum technique, both in the field of electrical discharge tubes, more especially electron tubes, and in the field concerned with incandescent lamps, there have already been Patented July 26, 1966 used for a relatively long time exclusively those heater elements or incandescent elements of refractory metal, the material of which is a refractory metal leading to a largecrystalline structure (frequently also called coarse-crystalline or long-crystalline structure), that is to say, a refractory metal wire which has a large-crystalline structure in its secondarily, i.e. completely recrystallised, state, respectively, condition. The methods of manufacturing wires of this type are known from numerous literature references and patent specifications. An exception from the general use of such large-crystalline wires is represented for example by the incandescent cathodes of transmitter tubes or valves. It is known that these incandescent cathodes consist of tungsten wires with a comparatively high (about 2%) content of thorium dioxide, these wires being of a small-crystalline structure and presenting very good electron-emitting properties at high operating temperatures above 1600 C. It is also known to persons skilled in the art and is for example described at pages to 137, 157 and 158 of the 1953 edition of the work entitled Tungsten lby Smithells that tungsten wires having a large-crystalline final structure are produced from such starting material to which certain additives are intentionally added during the course of their manufacture, which additives are almost completely removed from the material during the processing of the starting material to form the wire, so that the final tungsten wire only shows a content of foreign substances i.e. additives and impurities, which can scarcely be accurately determined numerically and which is of the order of magnitude of not more than only about 0.001% by weight. It is also known that tungsten wires manufactured with the use of such additives, after attaining their large-crystalline structure, show more favourable mechanical properties than wires having a structure of .the small-crystallin type. The large- -crystalline structure is achieved in the case of the aforesaid wires, when incorporated in incandescent lamps as the incandescent filament thereof, for example as the result of the incandescent lamp being switched on at the very first time. On being thus switched on, the tungsten wires are in fact heated to temperatures which are higher than their secondary recrystallisation temperature of more than 1800" C., but usually of about 2200 C. On the other hand, those wires which as a consequence of their manufacturing process still contain additives in relatively large quantity in their fully recrystallized state and thus have a small-crystalline structure after the secondary recrystallisation thereof, i.e. in their fully recrystallized state are known to show less favourable mechanical properties than wires having a large-crystalline structure. It is to be noted in this connection that the expression secondary recrystallisation in the present specification and claims means that recrystallisation, as a result of which the final crystal structure of the wire is developed. In the case of tungsten used in the vacuum art, the temperature of this secondary recrystallisation is between approximately 1900 to 2200 C., whereas the primary recrystallisation is already initiated at substantially lower temperatures of for example about 1000 C.

It is known that in the overwhelming majority of electron tubes the heater elements of their indirectly heated cathodes generally operate at such a temperature which is lower than the secondary recrystallisation temperature of the wires concerned. In the course of my investigations, I have now established the surprising fact that in all those fields of application in which the operating temperature of the heater elements of the cathodes is below their secondary recrystallisation temperature, those wires which would present a large-crystalline structure after their completed secondary recrystallisation do not, as a rule, present those favourable mechanical properties which would satisfy upto-date requirements, and accordingly the tubes provided with such cathodes usually also do not have the required long effective life.

In accordance with my invention, I have noW discovered that considerably better results can be obtained by using as heater elements of the indirectly heated cathodes such types of refractory metal wires which would present a small-crystalline structure in the event of an operating temperature above their secondary recrystallisation temperature, and which contain, even in their fully recrystallized state an inhibiting refractory oxide in a quantity amounting to at least 0.5% by weight. Such Wires which consist of a refractory metal and acquire a small-crystalline structure in the event of being heated to their secondary recrystallisation temperature have already long been known and used for other purposes. In general, these are wires which contain added substances which remain in them during the course of their manufacturing process, these additives being usually such refractory oxides which are difficult to reduce, such as thorium oxide, zirconium oxide, hafnium oxide, aluminum oxide, beryllium oxide, etc. I have however discovered as a result of my investigations that it is just those wires which contain one of the aforesaid oxides in the aforesaid quantity which are best suitable for .the uses according to the invention.

The invention is thus based on the fundamentally new discovery and my experiences acquired on the basis of this discovery, that contrary to the quite generally accepted former opinion and practice, it is more preferable, in the case of all such products used in the vacuum art, in which the heater elements work at an operating temperature below their secondary recrystallisation temperature, that is to say, below approximately 1900 to 2200 C. with tungsten, to employ those wires which would present a smallcrystalline and not a large-crystalline structure after their secondary recrystallisation, that is to say, contain added substances which remain in and do not escape from the wires. It also follows from the above discoveries that great care must be used during the manufacture of the heater elements of the cathodes according to the invention to ensure that the heater elements, after their having been manufactured as far as to be ready for their being mounted into the cathodes, are not treated at temperatures which are above their secondary recrystallisation temperatures.

The indirectly heated cathode according to the invention is thus characterised by the fact that the heater element thereof consists of a refractory metal wire which contains at least 0.5% by weight of a refractory oxide other than the oxide of said refractory metal but which only shows a primarily recrystallised structure, that is to say, a metal wire which has still not been completely (secondarily) recrystallised. As is known, the existence of this crystal structure of the wires of the heater elements can always be satisfactorily ascertained by means of microscopic examination.

As already mentioned, wires of refractory metal and containing refractory oxides, for example tungsten wires containing ThO have long been known, but have never been used for the purposes proposed by me according to the invention, i.e. as heater elements, but have been and still are employed as directly heated cathode-s of transmitter tubes or valves under quite different operating conditions, namely, at those operating temperatures which are above their recrystallisation temperature range. Furthermore, the new heater element wires are inactivated and only contain the added substances for the purpose of improving their mechanical properties.

I have experimentally established without any doubt that the leaking currents of the heater elements of the cathodes according to the invention are not in any way larger than those of the known cathodes, and that the new heater elements can also be employed without being provided with an insulating coating on their surface.

Although the cathodes according to the invention can be manufactured with heater elements of many kinds,

they are advantageously made with heater elements consisting of a tungsten wire, which wire contains for example at least 0.5% by weight of thorium oxide and has not been subjected to secondary recrystallisation. In this connection, it is to be noted that when the wires of the heater elements of the cathodes according to the invention are subsequently heated beyond their secondary recrystallisation temperature, the final small-crystalline structure is developed therein, which fact is likewise suitable for the identification of the heater elements of the cathodes according to the invention.

The surprisingly good properties as regards resistance to impact and vibration of the heater elements of the cathodes according to the invention are probably to be attributed to the fact that the refractory oxides present therein are inhibitive for the plastic deformations taking place within and between the crystals of the wires as a consequence of their being in situ in the wire. The excellent mechanical proper-ties of the heaters of the cathodes according to the invention have been checked by me in connection with manufactured radio tubes or valves, using the drop-test effected in rotating wheels, conventional for such tests, and by comparison with other normal commercial tubes. I have in this way ascertained that the mechanical resistivity of the heaters of the cathodes according to the invention exceeded that of the usual known cathode heaters by at least one order of magnitude. For example, radio tubes of the type UBL21 which are manufactured by the assignee of my present application and which were provided with the hitherto usual heaters of their indirectly heated cathodes, have been tested by me under such strict conditions that often 30 to 50% of the heater elements broke. The comparative tests carried out with tubes of completely identical structure but incorporating the cathodes according to the invention showed a percentage of breakages of the heater elements which was always below 1% and frequently even reached the value of zero, when tested under the same testing conditions as above. The heater elements consisted in both cases of single-coil tungsten wires carrying an aluminum oxide coating sintered thereon, the wire diameter being 45,u.. The wire material in the heater elements hitherto employed was the known tungsten wire practically free from foreign substance and not subjected to secondary recrystallisation, this being one which would present a large-crystalline structure in its fully recrystallised state. As regards the new heater elements, the pure tungsten oxide used as a starting material for their manufacture had admixed therewith thorium oxide in a quantity of 0.75% by weight at the time the said heaters were manufactured, during which manufacture the maximum temperature of the thermal treatment of the heater element did not exceed about 1550 to 1600 C. Apart from the thorium oxide, the wires of the heater elements of the new cathodes did not contain foreign substances in a quantity capable of being detected, and had a primarily recrystallised crystal structure.

The important improvement of the cathodes possible according to the invention permits electron tubes also to be used in those fields of application where it was only possible hitherto to employ transistors on account of their greater mechanical resistance. The use of the new cathodes is especially advantageous in connection with those high-vacuum electron tubes of which the dimensions are very small, and/or the wire of the heater element of their cathodes is very thin.

The heater elements of the cathodes according to the invention, together with the new cathode and an electron tube in which the new cathode is used, are illustrated on the drawing in some examples to be described hereinafter, it being understood that these examples are only illustrative and not to be interpreted in a limiting sense.

FIG. 1 shows, in a highly enlarged scale, a new heater element of the conventional hairpin type, with its intermediate parts broken away, the upper part of the figure being a longitudinal section and the lower part a side elevation.

FIG. 2 shows, also in a highly enlarged scale and in side elevation, with part broken away for the sake of clear illustration, a cathode according to the invention, and FIG. 3 is a longitudinal section of an electron tube provided with a cathode according to the invention.

The new heater element shown on FIG. 1 consists of a tungsten wire 1, bearing a sintered insulating coating 2 of alumina. The wire 1 has a thorium oxide content of 0.65 percent by weight, contains no other nOntungsten ingredients of measurable quantity and has an only primarily recrystallized structure. The coating 2 may be of any conventional type, but also of the type described and claimed in the co-pending application No. 861,383, now abandoned, of the assignee of the present application. This heater element, when mounted into any suitable cathode body, provides an indirectly heated cathode according to the invention.

The cathode shown on FIG. 2 has a tubular cathode body 3 consisting of any suitable conventional nickel alloy or nickel and bearing on its surface a coating 4 of any suitable electron-emitting material, such as a mixture of barium oxide, strontium oxide and calcium oxide. The tungsten wire whose straight ends 5 and 6 protrude from the cathode body on its lower end, has a coiled shape and bears the insulating coating 7 on its coiled parts positioned inside said cathode body 3.

The electron tube shown on FIG. 3 is a triode of conventional type, provided with a cathode according to the invention. Inside the bulb 10 there is an anode 11, a grid electrode 12, an indirectly heated cathode body 13, an electron-emi-tting coating 14 on said body, and a heater element 15 inside said body. The heater element 15 consists of a coiled tungsten wire of the kind described in connection with FIGS. 1 and 2 and having the shape shown on FIG. 2. The leading-in wires of this new heater element are denoted with 16 and 17 and their con-tact pins with 18 and 19.

References Cited by the Examiner UNITED STATES PATENTS 1,082,933 12/1913 Coolidge -176 X 1,461,140 7/1923 Ramage 75-176 X 2,098,807 11/1937 Jonker et al 313. 341 X 2,108,544 2/1938 Meyer 313-337 2,477,601 8/1949 Hanson 313-341 2,919,373 12/1959 Riley et a1. 313-341 X 2,948,609 8/1960 Millner et al 75-207 X 3,075,120 1/1963 Schniztel 313-341 X OTHER REFERENCES Pages 215228, 1955, Tungsten, by Li and Wang, published by Reinhold Publishing Corp., New York, 3rd ed.

Tungsten, C. J. Smith'ells, 3rd ed., Chapman and Hall Ltd., London; 1952, pages 136, 137 cited.

Materials and Techniques for Electron Tubes, Walter H. Kohl; General Telephone and Electronics Series, Reinhold Publishing Corp., New York, 1960, pages 299, 300, 311, 319, 86, 87 and 635 cited.

DAVID J. GALVIN, Primary Examiner.

RALPH G. NILSON, ARTHUR GAUSS, ROBERT SEGAL, GEORGE N. WE'STBY, Examiners.

C. O. GARDNER, Assistant Examiner. 

